Various technical fields use automatic or semiautomatic machines with tools mounted to the free end of an operating arm, which is movably attached to a support of the machine and moves with a single degree of freedom.
These tools shall be caused to contact an object upon which they are designed to operate, while ensuring that no excessive contact forces are generated between the tool and the object, to prevent any damage during such contact.
For this reason, it is highly advantageous to use detection means to detect the contact between the tool and the object to be acted upon, which detection means generally controls drives of additional automatic members that shall be only actuated when contact has occurred between the tool and the object.
A further very useful feature is the possibility of controlling the stroke and position of the arm and to cause it to reach a predetermined limit stop position which typically coincides with the contact between the tool and the object or with a predetermined position of the arm, to control the operation of the automatic members which, as mentioned above, shall be only operated once such limit stop position has been reached.
The tool supporting arm is typically driven by a double-acting fluid-dynamic actuator, which is interposed between the arm and the support of the machine in which the arm slides.
In order to obtain the above described features, the operation of the actuator must be controlled to limit the sliding speed of the piston in the jacket and the drive force that will be exerted on the object as contact occurs between the latter and the arm-supported tool.
Two possible alternative solutions are used in the prior art to control the arm stroke, and hence the action of the actuator.
In a first solution, one or more proximity sensors (selected from those known in the art) are mounted to the machine support, to the arm and, when needed, also to the tool.
Thus, a warning signal, typically an electric or pneumatic signal may be used to detect when the arm reaches the predetermined limit stop position, which signal is transmitted by the sensors to one or more control units of the machines, which are designed to control the operation of the automatic members.
Nevertheless, this first solution suffers from certain drawbacks.
A first drawback is that the areas in which the proximity sensors shall be mounted and positioned must be located and prepared on the arm support and possibly on the tool.
Furthermore, these operations shall be carried out in a particularly accurate manner, to avoid any inaccurate arm position detection.
A second drawback is that means have to be provided for transmitting and carrying signals from the sensors to the control unit, such as cables, pipes, optical fibers, and this involves an increase of machines' manufacturing costs, because in addition to sensor costs, seats have to be formed, by suitable processing, on the arm, the support and possibly the tool, for mounting such sensors.
Furthermore, if the sensor mounting positions are not easy to reach, the sensors must be calibrated according to their non-optimal location with respect to a limit stop position or the end position that the arm is required to reach.
A further drawback is that the sensors that are designed to generally detect a contact between a tool and an object upon which the latter is designed to act are typically mechanical sensors and hence clearances may exist between components that might cause inaccurate detections with respect to the positions that have been reached by the tool relative to the object.
Therefore, for improved accuracy, high-precision sensors must be used, which will have a proportionally higher cost.
A further drawback is that the lines that carry the signals between the sensors and the control unit are often mounted, at least partially, in the proximity of the object and this increases the risk of contact with the object or with parts that might come off therefrom during further processing.
In the second solution the arm is still moved by means of an actuator, but detection of a predetermined limit stop position of the arm occurs by measuring changes in the force of the actuator, namely by measuring the pressure thereof, which considerably increases above the normal operating value when the actuator is under a stress, because the arm and the tool have reached the limit stop position that stops any further sliding movement although the pushing action of the actuator continues.
Such second solution provides an advantage over the first solution, in that the lines required transmitting the signals indicating that the arm has reached its limit stop position or a predetermined position, are the lines that are used to connect the actuator to fluid source, also known as power source.
According to the second solution, a further parameter may be also used to detect that the arm has reached a limit stop position, i.e. the change in the energy (typically electric energy) absorbed by the actuator during normal operation and when the limit stop position has been reached; when this state is reached, energy absorption considerably increases due to the resistance opposed to the actuator.
Therefore, in short, this second solution is generally preferable, as long as pressure forces and changes thereof can be measured as the arm moves in its support.
Furthermore, this second solution has a much simpler construction than the first solution and hence, a lower cost and improved reliability with time.
However, this second solution also has various drawbacks.
A first drawback is that the speed of the arm relative to its support has to be controlled and limited, both to prevent kinetic reactions from causing the arm to slide beyond a predetermined end position with no limit stop abutment, and to allow accurate detection of the changes in pressure or (electric) force absorbed by the actuator.
Furthermore, the thrust imparted by the actuator shall be detectable by reading a single physical quantity and should not require comparison of two different quantities, which would involve the use of more advanced electronic systems and mapping thereof, requiring particularly expensive components and accordingly expensive calibration thereof.
Even when using constant voltage-controlled electromechanical actuators, in which the current absorbed by the actuator provides a reliable reading of the force exerted thereby, absorbed current detection requires high cost sensors, as well as high cost control unit components to analyze the absorbed and detected current value.
Furthermore, when the arm has a substantially straight motion, the electromechanical actuators designed to impart this motion are also costly, as they are typically composed of a rotary motor and a rack or recirculating-ball screw transmission.
If constant-flow hydrostatic actuators are alternatively used, in which the hydrostatic fluid pressure in the pressure chamber is directly proportional to the force exerted by the actuator, other drawbacks occur.
A first drawback of this solution is that hydrostatic actuators require a hydraulic station to supply hydraulic pressure, even in machines that would not require it as such, such as most light industrial automation machines, typically operated by compressed air.
Furthermore, the hydraulic circuit has to be controlled by solenoid valves which in turn require an electronic controller to be connected thereto, thereby increasing construction costs.
In addition, the detection of hydraulic pressure in the pressure chamber of the actuator requires electronic pressure transducers, which are also expensive, as well as expensive electronic components for interpreting the signals generated by these transducers.
Finally, a further drawback of the prior art in general is that an operator is required to manually hold the actuation control to move an operating arm to a predetermined position.